Update to post-then-commit flow for purchases

Just like return invoices currently do.

Create an Avatax "SalesOrder" to capture tax info prior to the "confirm" step.
At the "confirm" step create an uncommitted Avatax "SalesInvoice" to capture tax information.
Then after the order is complete commit the existing SalesInvoice.

Also:

- Provide a DB migration to create uncommitted SalesInvoice records for existing orders in the "confirm" state
- Remove now-unused "avalara" gem
- Remove obsolete "endpoint" config